a) Description of the Related Art
The present invention relates to an optical head assembly for recording on and reproducing from optical recording media such as CDs (compact disks) and DVDs (digital video disks).
b) Description of the Related Art
An optical head assembly converges a laser light emitted from a laser light source as an optical spot onto a recording surface of an optical recording medium through an objective lens and detects a returning light reflected upon the optical recording medium through an optical detector to reproduce data recorded on the optical recording medium. During the above operation, the optical head assembly also controls the position of the objective lens in the tracking direction so that the laser light emitted from the laser light source completely scans the track of the recording surface of the optical recording medium, and further controls the position of the objective lens in the focusing direction so that the laser light focuses on the recording surface.
To perform these tracking and focusing controls, the optical data corresponding to tracking errors and focusing errors of the laser light are detected by using the returning light from the optical recording medium.
In general, a three-beam method is employed for detecting tracking errors. With the three-beam method, the laser light is split by a diffraction grating into three beams in the track direction of the recording surface and the like. Of the three beams, the +1st-order beam and -1st-order beam converge at the edge portions of the track. Tracking error signals are generated based on the energy difference between the +1st-order beam and -1st-order beam reflected at the edge portions of the track.
Conventionally an optical system is designed, based on the track pitch of the optical recording medium, for converging the +1st-order beam and -1st-order beam on the recording surface of the optical recording medium. It is designed such that the +1st-order beam and -1st-order beam converge at the edge portions of the track and the phase difference between the +1st-order beam and -1st-order beam is at a maximum in order to obtain the tracking error signals based on the energy difference between +1st-order beam and -1st-order beam.
In conventional optical head assemblies in which the optical system is designed based on the track pitch of the optical recording medium, however, if the track pitches on the recording surfaces are different, the +1st-order beam and -1st-order beam cannot be converged at the edge portions of the track and also the phase difference between the +1st-order beam and -1st-order beam does not reach the above mentioned maximum state (i.e., does not satisfy the above relationship). Therefore, when the track pitch is 1.6 .mu.m for CD and 0.74 .mu.m for DVD, the tracking error signals cannot be detected by a common optical system.